Cosmetics can comprehend all measures which, for aesthetic reasons, bring out changes on skin and hair or are used for washing the body. Cosmetics thus means to care for, to improve and to beautify the outside of the body in order, through vision, touch and smell, to please those around us and also ourselves. Even thousands of years ago, cosmetics were used by people for this purpose. Colour was applied to lips and face, valuable oils were used for anointing and scented water was used for bathing.
Whereas the cleaning effect of a shampoo, the improvement in combability of a hair rinse or the curling of the hair as a result of a perm can be established easily and objectively, other effects and properties of cosmetic products are virtually immeasurable or are only very noticeable to the individual. These include, for example, a certain (invigorating, soft, supple, smooth etc.) feel on the skin or the softness and suppleness of the skin following the application of a cosmetic, and the fullness and bounce of the hair and the like. In addition, the consumer's expectation is also governed by secondary properties of the product. These are, in particular, the scent and the colour of the cosmetic, and also its packaging, the price, the manufacturer and the advertising.
One property of cosmetic products which is very important for the consumer but which can only be quantitatively measured with difficulty is its texture. The term “texture” is understood as meaning those properties of a cosmetic which relate to the structure of the preparation, are perceived by the sense of touch and in some cases can be expressed in mechanical or rheological flow properties. The texture can, in particular, be tested by means of sensorics. The texture of cosmetic products, which can, if appropriate, be influenced using additives, is of virtually equal importance to the consumer as their effects which can be established objectively.
The term “sensorics” refers to the scientific discipline which deals with evaluating cosmetic preparations on the basis of sensory impressions. A sensory assessment of a cosmetic is made by reference to visual, olfactory and haptic impressions.                Visual impressions: all features which can be perceived by the eye (colour, shape, structure).        Olfactory impressions: all odour impressions which can be perceived by drawing in air through the nose, which can often be differentiated into initial odour (top note), main odour (middle note, body) and after-odour (finale). The volatile substances which are only released upon application also contribute to the olfactory impression.        Haptic impressions: all sensations of the sense of touch, which relate primarily to structure and consistency of the product.        
Sensory analysis makes use of the possibility of integrally acquiring the overall sensory impression of a product. Disadvantages of sensory analysis are the subjectivity of the impression, the fact that the test persons can easily be influenced and the considerable scattering of results caused by this. These drawbacks are nowadays countered by using groups of trained test persons, screening the testers from one another, and statistical evaluation of the in most cases numerous analysis data.
The method of sensory analysis used most often in research and development is the difference test. The task here is usually restricted to recognizing one of several samples or sample differing from a control sample. Whereas with difference tests within one test only two samples are compared with one another, with the ranking test a series of three or more samples is to be determined, usually according to intensity, quality, popularity or similarity to a comparison sample. This (simple) method is suitable, for example, for a pre-selection of samples in product optimization and is also often used in market research.
Alternatively, the texture of a cosmetic preparation can also be characterized by reference to its rheological properties.
The term “viscosity” is understood as meaning the property of a liquid to offer resistance (tenacity, internal friction) to the mutual laminar displacement of two adjacent layers. This so-called dynamic viscosity according to η=τ/D is defined as the ratio of the shear stress to the rate gradient perpendicular to the direction of flow.
Whereas a graphical representation of the flow behaviour of Newtonian liquids at constant temperature produces a straight line, in the case of so-called non-Newtonian liquids, considerable deviations are often evident, depending on the particular shear stress τ. In these cases, the so-called apparent viscosity can be determined which, whilst not obeying the Newtonian equation, can, however, be used to ascertain the true viscosity values through graphical methods.
The term yield point is understood as meaning the smallest shear stress above which a plastic material behaves in rheological terms like a liquid (DIN 1342-1: 1983-10). The yield point is determined by plotting a flow curve (DIN 53019: 1980-05; DIN 53214: 1982-02). The value obtained depends greatly on the time scale (strain rate) on which the measurement is based. The flow curve is a graphic representation of the relationship between shear stress and rate gradient D for a liquid subjected to lamellar flow or for a plastic material above the yield point. Flow curves are usually measured in rotary viscometers. In the case of speed-controlled systems, while presetting a continuously or step-wise varied rate gradient, the resulting torque is measured and the shear stress proportional to this is calculated. In the case of shear stress-controlled systems, the reverse is true. From the flow curve it is possible to calculate the viscosity as a function of the rate gradient, to determine yield points and to characterize the flow behaviour.
Cosmetic O/W formulations always have pseudoplastic flow behaviour with greater or lesser marked thixotropy which makes it easier for the consumer to apply such products. Furthermore, the extent and nature of the pseudoplasticity and also the thixotropy also determine the distribution on the skin and the feel of the skin after rubbing in because, for example, a thixotropic O/W formulation can, after spreading, again build up a (feelable) structure on the skin.
The person skilled in the art is of course aware of a large number of options for formulating stable O/W preparations for cosmetic or dermatological application, for example in the form of creams and ointments which are spreadable in the range from room temperature to skin temperature, or as lotions and milks, which are more flowable within this temperature range.
O/W emulsions for (large-area) application within the bodycare sector—i.e. O/W emulsions which, in particular, are to be removed from customary plastic bottles in a relatively large amount—are usually formulated so that they can be spread easily on the skin and at the same time allow good emptying of the last bits from the packaging. In order to be able to satisfy both criteria, the O/W emulsions of the prior art must accordingly have a rather low viscosity of at most 6000 mPa·s (determinable using a Haake viscotester VT-02 at 25° C.). This requirement placed on a corresponding skincare product leads accordingly always to identical or similar formulations.